NOx storage-reduction catalysts have come to be used practically as catalysts for purifying exhaust gas from lean-burning engines. In these NOx storage-reduction catalysts, NOx are stored in a NOx storage material when engine operation is on the fuel lean side, by pulsed control of the air-fuel ratio such that the air-fuel ratio is varied from the fuel lean side to the stoichiometric to fuel rich side. The stored NOx is then released on the stoichiometric to rich side and purified by reacting with a reducing component such as HC or CO due to the catalytic action of a precious metal.
Example of a NOx storage-reduction catalyst usually include a catalyst in which a catalyst layer containing a catalytic metal such as platinum (Pt), palladium (Pd) or rhodium (Rh) formed on a honeycomb substrate made of cordierite and the like and a NOx storage material containing an element such as an alkaline metal, alkaline earth metal or rare earth element supported on the catalyst layer. In addition, potassium compounds and barium compounds are typically used for the NOx storage material from the viewpoint of their high NOx storage performance.
Japanese Unexamined Patent Publication No. 2002-191989 discloses a NOx storage-reduction catalyst in which a poorly water-soluble hydrogen tartrate salt (potassium hydrogen tartrate) is used as the NOx storage material and the NOx storage material is only supported on catalyst support particles having a high specific surface area, and thereby preventing the elution of the NOx storage material and the migration thereof to catalyst support particles having a low specific surface area when exposed to high temperatures.